Reportable Segments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Rental income:
Same store	$ 465,653,000	$ 443,152,000
Non-same store/other	71,349,000	1,232,000
Total rental income	537,002,000	444,384,000
Operating expenses:
Same store	166,456,000	161,767,000
Non-same store/other	31,763,000	6,964,000
Total operating expenses	198,219,000	168,731,000
NOI:
Same store	299,197,000	281,385,000
Non-same store/other	39,586,000	(5,732,000)
Total NOI	338,783,000	275,653,000
Total assets	23,795,158,000		17,201,000,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	26,599,000
Reconciliation of NOI
Rental income	537,002,000	444,384,000
Cost of Property Repairs and Maintenance	(107,083,000)	(92,952,000)
Real estate taxes and insurance expense	(68,647,000)	(52,440,000)
Property management expense	(22,489,000)	(23,339,000)
Total operating expenses	(198,219,000)	(168,731,000)
Reportable Segments (Textuals) [Abstract]
Units in same store properties	90,350
Northeast [Member]
Rental income:
Same store	186,839,000	178,867,000
Non-same store/other	43,702,000	404,000
Total rental income	230,541,000	179,271,000
Operating expenses:
Same store	69,299,000	66,752,000
Non-same store/other	14,060,000	907,000
Total operating expenses	83,359,000	67,659,000
NOI:
Same store	117,540,000	112,115,000
Non-same store/other	29,642,000	(503,000)
Total NOI	147,182,000	111,612,000
Total assets	11,571,120,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	9,030,000
Northwest [Member]
Rental income:
Same store	102,570,000	95,144,000
Non-same store/other	17,389,000	722,000
Total rental income	119,959,000	95,866,000
Operating expenses:
Same store	35,427,000	34,662,000
Non-same store/other	6,667,000	381,000
Total operating expenses	42,094,000	35,043,000
NOI:
Same store	67,143,000	60,482,000
Non-same store/other	10,722,000	341,000
Total NOI	77,865,000	60,823,000
Total assets	4,745,161,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	8,137,000
Southeast [Member]
Rental income:
Same store	64,678,000	61,778,000
Non-same store/other	256,000	0
Total rental income	64,934,000	61,778,000
Operating expenses:
Same store	24,533,000	24,064,000
Non-same store/other	104,000	0
Total operating expenses	24,637,000	24,064,000
NOI:
Same store	40,145,000	37,714,000
Non-same store/other	152,000	0
Total NOI	40,297,000	37,714,000
Total assets	1,707,027,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	3,799,000
Southwest [Member]
Rental income:
Same store	111,566,000	107,363,000
Non-same store/other	8,100,000	87,000
Total rental income	119,666,000	107,450,000
Operating expenses:
Same store	37,197,000	36,289,000
Non-same store/other	3,397,000	22,000
Total operating expenses	40,594,000	36,311,000
NOI:
Same store	74,369,000	71,074,000
Non-same store/other	4,703,000	65,000
Total NOI	79,072,000	71,139,000
Total assets	3,925,524,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	3,993,000
Other [Member]
Rental income:
Same store	0	0
Non-same store/other	1,902,000	19,000
Total rental income	1,902,000	19,000
Operating expenses:
Same store	0	0
Non-same store/other	7,535,000	5,654,000
Total operating expenses	7,535,000	5,654,000
NOI:
Same store	0	0
Non-same store/other	(5,633,000)	(5,635,000)
Total NOI	(5,633,000)	(5,635,000)
Total assets	1,846,326,000
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets	$ 1,640,000